The present invention relates to a fixing apparatus to be used for an image forming operation employing the electro-photographic method and an image forming apparatus provided with the fixing apparatus mentioned in the above.
In the image forming apparatus that employs the electro-photographic method, an image is formed on a recording medium by conducting the steps of: forming a latent image on a photoreceptor member; developing the latent image with toner; transferring the toner image formed on the photoreceptor member onto the recording medium; and fixing the transferred toner image onto the recording medium by applying heat and pressure to both the toner image and the recording medium.
The fixing operation is achieved in such a manner that the recording medium bearing the toner image is introduced into a nip portion formed between a heating member and a pressing member, so as to fuse the toner image at the nip portion by applying heat and pressure. Accordingly, a resin of tetrafluorethylene-perfluoroalkyl vinyl ether copolymer (PFA), having both release and heat-resisting properties, is frequently employed for a surface layer of the heating member, which directly contact the toner image.
In the image forming apparatus that forms a toner image including a relatively large amount of toner, such as a full color toner image or the like, it is desirable that a flexible surface layer is formed on the heating member, so that the surface layer of the heating member flexibly deforms corresponding to surface unevenness formed by the protrusions of toner image, so as to uniformly heat the toner image.
Patent Document 1 (Tokkai 2005-163837, Japanese Non-Examined Patent Publication) sets forth the PFA, having a measuring value of elastic modulus (G′) being equal to or smaller than 60 MPa, measured at 100° C. by the dynamic viscoelasticity measuring apparatus (ARES), as the surface layer that complies with the condition mentioned in the above.
According to the fixing operation employing the heating member having the surface layer set forth in Patent Document 1, since the uniform fixing operation is conducted allover the image, it becomes possible to form a high quality color image on the recording medium.
To control the surface temperature of the heating member and/or to prevent the heating member from overheat, the fixing apparatus is usually provided with a temperature sensor to detect the continuous transition of the surface temperature of the heating member and/or anther type temperature sensor, such as a thermostat, a thermal fuse, etc., to detect the fact that the surface temperature exceeds a predetermined value.
Although both contact type and noncontact type temperature sensors are available in the market, a contact type temperature sensor is frequently employed to reduce the cost of the apparatus. Accordingly, the contact type temperature sensor is usually employed in the apparatus, except that the noncontact type temperature sensor is selectively employed at a paper sheet passing region in which the heating member contact the recording medium.
Although a good fixing operation can be achieved by forming the flexible and deformable surface layer on the surface of the heating member as mentioned in the above, when the heating member is activated while the temperature sensor is made to contact the flexible surface layer, it has been revealed that various kinds of defects, such as an occurrence of abnormal sounds, breaking of a wire coupled to the temperature sensor, an increase of driving torque for driving the heating member, etc., could be generated, since the temperature sensor interlocks with the surface layer due to the deformation of the surface layer, caused by the close contact between the temperature sensor and the surface layer of the heating member.